1. Field of Invention
This invention relates to the formation of holes of relatively large diameter in soils such as silty clays, which are of such a composition and density as to be susceptible to compaction or displacement by application of a high intensity ramming force in relatively small increments.
2. Description of the Prior Art
Large diameter shafts on the order of 2 ft. to 15 ft. or more in diameter are commonly formed in such soft soils by sinking an open ended cylindrical steel shell or casing. Earth is removed from within the causing by means of an auger type excavating device or a grab type (clam shell) excavator. Vertical ramming force, occasionally with the addition of rotational, oscillatory, or vibratory forces, is often necessary to force the casing down into the soil. The soil presses tightly against the outer surface of the casing due to soil displacement and compaction. This increases frictional forces acting on the outer surface of the casing and makes downward moving of the casing difficult.
Underreaming tools may be employed in an effort to overcome the severe frictional forces tending to resist downward movement of the casing. These tools remove soil from beneath the lower edge of the casing and so remove resistance of the soil against this lower edge. But unless the soil is virtually self-supporting, friction will again build up along the outer surface of the casing resisting its downward movement.
In self-supporting soil, such as very stiff clay, the casing may be unnecessary. However, because soil density varies greatly and is of indeterminate quality, the reliability of this method is suspect. Personnel are not permitted to enter an uncased hole due to the danger of partial collapse of the hole. The uncertain results of this method, together with the attendant expense of loss of construction time in the event of a partial collapse, must always be considered as a possibility.
Present methods described above require massive equipment. A disposal cycle auger-type excavator, i.e., one which is raised out of the hole periodically to spin-off excavated soil, requires from 20 H.P. to 30 H.P. per foot of hole diameter and is typically capable of excavating holes up to six feet in diameter to a maximum practical depth of about 100 ft. In easily augered self-supporting soil, production rates up to 20 ft./hour may be achieved. Where casing of the hole is required, as in unstable soil, equipment requirements are increased and production rate greatly reduced. The cost of disposal cycle auger-type excavator equipment ranges from $60,000 to $150,000 and up.
Grab type excavators with which a shield is invariably employed, require somewhat less horsepower but achieve lower production rates. Equipment cost, including shield placing equipment, is somewhat greater than disposal cycle auger-type excavators. While both types of equipment are capable of penetrating very dense soil strata including those containing some cobbles and boulders and even low strength rock, they are unsuited to placing large diameter holes through soft soil at great depth, say on the order of 100 ft. and over. This unsuitability stems from uneconomically high power requirement, and from the progressively greater time and cost required for soil removal, as hole depth increases.
With the aid of pre-drilling techniques, very heavy steel casings can be driven to great depths through soil. These techniques are used in the construction of oil well drilling platforms in deep water. Equipment for driving these tubular casings up to 42" in diameter to depths of 400 ft. weigh as much as 300 tons and cost on the order of $1,000,000 and up. Equipment for driving such casings to depths of 1,000 ft., which will be required in the near future, is not available commercially at this time.